1. Field of the Invention
The present invention relates generally to semiconductor processing and, more particularly, to photoresist patterning.
2. Description of Related Art
One of the processing steps used during manufacturing processes for fabricating transistors, conductive lines and via is photolithography. Photolithography is used numerous times during typical manufacturing processes and is one of the more important as well as one of the more limiting processes for determining a maximum density and final reliability of a given integrated circuit. Photolithography is particularly important in positioning the transistors, interconnect layers and via and in ensuring their uniformity.
A typical photolithographic process is implemented by depositing onto a working surface, by means such as a spinner, a layer of photosensitive resist that can be patterned by exposure to ultraviolet (UV) light or another radiation type. The working surface may be a semiconductor wafer, interconnect layer or other layer depending on the current manufacturing stage of the integrated circuit. The photoresist layer is sensitive to light and may be patterned based on exposing the photoresist to a corresponding pattern of light.
When exposed to light, photoresist may either be hardened or softened, depending on the type of photoresist used. Positive photoresist, also known as light-softening photoresist, can be depolymerized by exposure to radiation such as UV light. Therefore, with positive photoresist, areas exposed to radiation are dissolved upon placement in a developer, while the masked, unexposed areas remain unaffected. On the other hand, negative photoresist, which is a light-hardening photoresist, can be polymerized by exposure to radiation, meaning that the exposed areas remain, while the covered areas are dissolved. Thus, depending on the type of photoresist utilized, the pattern transferred to the photoresist on the wafer is either a positive or a negative image of the photomask pattern.
To undergo exposure, the photoresist-covered wafer is placed beneath a photomask designed to prevent the penetration of radiation through certain portions of the photoresist. Predetermined areas of the photoresist then undergo a degree of polymerization or depolymerization, which can be a function of the nature and extent of photoresist exposure. The photomask forms the pattern by utilizing areas that block the light and other areas that allow the light to pass from the light source to the photoresist layer. The pattern of light created by the photomask is typically for a single die on a wafer. A lens may be positioned between the photomask and the photoresist layer to reduce the size of the pattern and to focus the pattern of light onto the die. A lithography tool steps from one die to the next die on the wafer and repeats the process until all selected die on the wafer have been exposed to the pattern of light created by the photomask.
A chemical bath known as a developer can then be used to dissolve parts of the photoresist that remain relatively depolymerized after the exposure by placing the wafer therein and allowing the wafer to be rinsed for a designated time period. Having received the pattern from the photomask, the layer of photoresist on the wafer is typically referred to as a layer of patterned photoresist. The presence or absence of photoresist across a working surface creates a pattern or template to be used by subsequent processing steps of the integrated circuit. For example, an etching or an ion implantation process may be used after the lithography step on the exposed areas without photoresist to continue the manufacturing process of the integrated circuit. In conventional methods, plasma etching is typically used to trim features of the post development patterns of photoresist. However, this approach may be unable to perform pattern selection. Using double exposure with strong phase-shifting masks (PSMs) may be implemented to reduce the dimensions of features of a photoresist pattern. The costs of using strong PSMs, however, can be relatively high.
A need thus exists in the prior art to efficiently shrink feature widths of patterned photoresist layers in a selective manner. A further need exists for shrinking feature widths of patterned photoresist layers in a more economical way than may be associated with for example strong PSM.